Electronic control systems often need to drive load devices. Transistor based load drivers that drive these loads must be designed to provide optimal drive capability while offering driver protection in case of an open or short circuit while operating over a wide thermal envelope. Certain types of loads must also be current regulated, so measuring actual load current is important for both protection of the transistor driver, as well as for optimal load control via regulation. Other applications need to measure load current for diagnostic purposes.
Conventional current sensing schemes include insertion of a relatively low resistance resistor in series with the load and the driver transistor. A voltage measured across the resistor is used to indicate the load current. A problem with this approach is that it requires a separate, and usually a relatively bulky component, which is by definition lossy in nature. This series connected resistor generates extra heat as it operates, which makes the driver circuit less reliable.
One loss-less load current sensing scheme applies a current transformer that is magnetically coupled to the load circuit. This scheme is costly, bulky, and is more difficult to manufacture because robust physical mounting of the magnetic current transformer is difficult at best--making it a poor choice in adverse vibration environments such as in an automotive application.
Another loss-less load current sensing scheme is to use a field effect transistor packaged with an integral current mirror. A problem with these devices is that the current mirror circuit requires a significant amount of die area. Furthermore, the current-proportional signal that these integral current mirror based devices provide have a built-in error that changes over the operating temperature of the device.
What is needed is an improved device for driving loads that includes loss-less current sensing that is more physically robust, more reliable, and easier to manufacture.